The present invention relates to a process for separating from each other materials dissolved in water by means of a cementation agent in the presence of substantial amounts of only such materials as have a hydrogen overpotential higher than the potential between the cementation agent and hydrogen gas, the cementation agent being less inert than hydrogen, and to a device for applying the process according to the invention. This invention particularly relates to a process for the cementation of cadmium by means of zinc.
The raw material generally used in an electrolytic zinc process is a sulfidic zinc concentrate, which is first roasted and then the roasted product is leached with return acid from the electrolysis and the obtained product is an impure zinc sulfate solution. This solution contains about 150 g of zinc/l and varying amounts of the components present in the concentrate. The purpose of the further treatment of the solution is to separate the zinc from these impurities In principle this is achieved in three stages. At the first stage, the solution is neutralized, in which case the hydroxides not as easily soluble as zinc hydroxide are precipitated and separated. At the second stage, the metals more noble than zinc are cemented from the solution by a zinc powder treatment according to the following reaction: EQU Me.sup.+.sup.+ + Zn .fwdarw. Me + Zn.sup.+.sup.+
At the third stage, the zinc is separated from the solution by electrolytic precipitation and the components less noble than zinc remain in the solution.
The number of components precipitating at the second stage is great, but the most important ones are usually copper, cadmium, cobalt, nickel, arsenic, and antimony. These must be removed quite completely, partly because they precipitate together with zinc in the electrolysis and thereby yield an impure product, and partly because they act as poisons in the electrolysis and develop hydrogen gas at the cathode in place of zinc precipitation. In addition, copper and cadmium are of an economic importance and they are refined further. Even though these components are considerably more noble that zinc, only copper, cadmium, arsenic, and antimony can be simply cemented by means of zinc powder alone. Cobalt and nickel can, however, be also cemented by using certain additives, and those must commonly used are arsenic and antimony, which are usually added into the solution as trioxides. While these additives accelerate the cementation of cobalt and nickel, they have an opposite effect on the cementation of cadmium. This is most likely because these elements lower the hydrogen overpotential on cadmium (as well as on zinc), in which case the reaction Cd + 2H.sup.+ .fwdarw. Cd.sup.+.sup.+ + H.sub.2 is catalyzed, which makes the cadmium more difficult to reduce into a metallic form; and even if this happens, the metal easily redissolves. This effect of arsenic and antimony on cadmium is also greatly strengthened at a raised temperature, while the cementation of cobalt and nickel is considerably facilitated. Two different methods are therefore used in processes in which arsenic or antimony is added to cement cobalt and nickel. In the first one, a low temperature (about 70.degree. C) is used, which makes it possible to precipitate all impurities simultaneously. This requires, however, a great zinc powder surplus to completely cement the cobalt and nickel as well as a relatively long reaction period. Also, by this method a precipitate is obtained which contains a great deal of metallic zinc and little cadmium; this increases the amount of solution in the cadmium process in which this precipitate constitutes the raw material and in which both the zinc and the cadmium are brought into solution.
In the second method, the above conformities to law are applied so that a high temperature (above 90.degree. C) is used for accelerating the cementation of cobalt and nickel, while the cadmium can be simultaneously kept almost completely in the solution. The cadmium can be then precipitated at a second stage in the absence of impurities which lower the hydrogen overpotential on cadmium. In this case a relatively low zinc powder surplus is sufficient and a precipitate is obtained which has a considerably better cadmium/zinc ratio than in the former method.
Even though in the second method the necessary zinc powder consumption is lower than in the first one and even though the obtained cadmium cementate has a good zinc/cadmium ratio in comparison with the first process, the zinc surplus is, however, usually about 500 % of the equivalent amount. In this case the requisite solution volume in cadmium production is also five times what would be theoretically necessary. This high zinc powder surplus is required partly for obtaining a moderate reaction period but also partly for keeping the Cd.sup.+.sup.+ content low enough in the solution (to prevent the reoxidation of cemented cadmium).
When cadmium is cemented from the "purified" solution described above, obtained, for example, by cementing all other impurities by the second method, the main reaction is as follows: EQU Cd.sup.+.sup.+ + Zn .fwdarw. Cd + Zn.sup.+.sup.+ (1)
This reaction follows the formula ##EQU1## in which -(dC.sub.Cd /dt) = cemented cadmium per time unit
k = reaction velocity constant PA0 A = surface area of zinc powder (amount of zinc powder) PA0 V = volume of solution (volume of reactor) PA0 C.sub.Cd = cadmium concentration at a given time PA0 A/V = concentration of zinc powder
Formula 2 indicates that with a certain A/V ratio a shorter reaction period is obtained in a batch process than in a continuous-working system (when C.sub.Cd .fwdarw. 0), when the average C.sub.Cd is higher in a batch process. In most zinc processes according to the second method, the batch process is the most common, and usually a compromise is made between the values A and V, so that the reaction period is about 1 - 11/2 hours, in which case the above-mentioned quintuple zinc powder surplus is required. The compromise leads to a reaction volume of about 600 m.sup.3 when the production is 100,000 tons of zinc a year. Formula 2 also indicates that if reaction 1 is carried out according to the countercurrent principle, adjustment must always be made to the effect that the additional expenses due to the zinc powder are balanced by a lower reactor volume when the A/V ratio rises.
The countercurrent principle is used to some extent in continuous-working processes according to the first method (AIME World Symposium on Mining and Metallurgy, 1970, Lead & Zinc, pp. 208-9 and 239). In this manner, the zinc powder consumption can be also diminished since the last stage, into which the zinc powder is added, has a low cadmium content and at that stage it is easier to obtain a sufficient zinc surplus for producing a pure solution. The gained advantages are, however, very small in conventional processes.